Lithium Formate for EPR Dosimetry (2): Secondary Radicals in X-Irradiated Crystals

The secondary radiation-induced radicals in lithium formate monohydrate were studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques complemented with periodic density functional theory (DFT) calculations. Single crystals of lithium formate monohydrate were X irradiated at 77 K and at room temperature. The main radicals present after irradiation at 77 K are the CO2 center dot- radical (R1), the recently identified protonated electron-gain product, HCOOH center dot- (R2) (Krivokapic et al., Radiat Res 2014: 181:503-11), and a different geometrical conformation of this latter radical, a species that, up until now, has remained unidentified (R3). The successful quantum chemical modeling of R3 confirmed its structure and also provided a possible mechanism for its formation. After irradiation at 295 K, the crystals were investigated both shortly after irradiation and after storage for eight months at room temperature in ambient environments. After long-term storage the CO2 center dot- radical had significantly decayed and the EPR spectra were dominated by two minority radicals. Both of these radicals are most likely formate-centered p-radicals, and based on the observed EPR parameters (g-and hyperfine coupling tensors) tentative candidates are the CO center dot- radical and the dimer formed by the CO2 center dot- radical and a neighboring formate molecule yielding the radical -O2C center dot O center dot(CH)-C-center dot center dot O-. (C) 2015 by Radiation Research Society